Melatonin promotes plant tolerance to abiotic stresses by stimulating the expression of many stress-related genes and protecting plants from oxidative stress, which has been reviewed extensively. Salinity forces plants to uptake excessive amounts of sodium (Na⁺) and chloride ions, resulting in the alteration of essential minerals such as potassium (K⁺), calcium, magnesium, and iron homeostasis. Moreover, exposure to essential nutrient deficiencies, such as nitrogen, iron, sulfur, and potassium changes ionic homeostasis in plants. This review highlighted the effects of melatonin on the improvement of plant Na⁺-K⁺ balance and micronutrient homeostasis under salinity conditions and on the improvement of ionic homeostasis in plants under nutrient-deficit conditions. Melatonin inhibits Na⁺ loading in roots and increases Na⁺ retrieval from shoots resulting in increased K⁺ accumulation. Melatonin mainly maintains Na⁺-K⁺ homeostasis by upregulating the Na⁺/H⁺ antiporter 1 and AKT serine/threonine kinase 1 transporter genes. Moreover, melatonin improves calcium, magnesium, copper, and iron contents in plants under stress conditions. Under nutrient-deficient stress, melatonin modulates numerous transporter genes and regulates the uptake and translocation of different essential nutrients. In-depth ionomics studies should be performed in plants exposed to salinity and nutrient deficiency stress to better understand the potential mechanism of melatonin-assisted ionic homeostasis under these conditions.